Method of injection molding a foamed resin product having a smooth surface involving surface heating of the mold by applying high current low voltage electric power

ABSTRACT

The invention comprises a method to quickly heat a mold cavity surface or portions thereof just prior to the injection of a foamable plastic resin. The apparatus comprises one or more thin metal surface sheets or bands of high electrical resistivity in the mold cavity covering those portions of the mold where a smooth surface finish on the product is desired. In an alternative form the surface sheets may be plated or sprayed metal of high resistivity on a layer of thin electrically non-conducting plastic in turn adhered to the mold cavity surface. 
     The thin metal surface sheet is resistance heated approximately 300° F. in a few seconds or less by a low voltage high current source of electric power just prior to the injection of the foamable plastic. The high current passing through the metal of high resistivity causes conversion of electrical energy into sensible heat very quickly to accomplish quick heating of the mold cavity surface sheet. The low voltage eliminates the need for substantial electrical insulation between the thin metal surface sheet and the bulk metal of the mold. Where steel molds and steel surface sheets are employed, the normal oxide coating on the steel serves as the insulation at the interface of the surface sheet and the mold. 
     To reduce or minimize arcing in the electrical switch gear employed to momentarily connect the high current low voltage source to the surface sheet, a water cooled electric heating pipe bypass is employed as an electrical energy sink during the portion of the molding process that does not employ heating of the surface sheet.

BACKGROUND OF THE INVENTION

The field of the invention pertains to injection molding of foamableplastic products and, in particular, to the provision of a smoothsurface finish on the foamed plastic products.

For aesthetic and functional reasons a smooth, shiny and blemish freesurface finish is desired on many foamed plastic products. Such a smoothand blemish free surface finish can be provided by molding with diesheated to a relatively high temperature (300° F.-550° F.) prior toinjecting the foamable plastic into the die cavity. The uppertemperature is typically selected to be above the heat distortiontemperature of the foamable plastic. The dies must then be cooled toapproximately 140° F. before the mold is opened and the product ejected.The repeated heating and cooling of the relatively massive bulk of themold requires considerable time and energy.

In order to substantially lessen the cycle time necessary to heat andcool the mold, means for heating only the cavity surface just prior toinjection of the foamable plastic have been developed. Exemplifying thisapproach in the prior art is U.S. Pat. No. 4,201,742. Disclosed is amethod for heating the mold cavity surface by means of condensing steamthereon and draining the condensate from the cavity just prior toinjection of the foamable plastic resin. This method provides a smoothsurface finish on the product as desired, however, despite much effortto overcome water marks and streaks caused by condensate near the bottomof the mold cavity, such defects continue to be a problem.

A method and apparatus for dry heating the cavity surface is shown inU.S. Pat. No. 2,979,773 wherein a semiconductive film is coated on themold surface and an aluminum electrode plated onto the semiconductivefilm. The mold body constitutes the other electrode. A heating currentpasses through the entire mold with the greatest temperature riseoccurring in the semiconductive film. Such a construction is limited torelatively short production runs because the aluminum and semiconductivefilm coatings are subject to rapid wear with repeated molding cycles.

U.S. Pat. No. 3,173,175 discloses a thin metal resistance element on orembedded in a glass or ceramic insulative layer. Glass and ceramiccoatings, however, severely limit the mold steels that can be used tosupport the glass and ceramic coatings and are very difficult to applyonly to those areas of the mold surface to be heated.

U.S. Pat. No. 2,984,887 discloses an electrically heated copper orsilver coating or preformed cup inserted in a ceramic mold. The mold isheated for a time sufficient to dry the skin on ceramic green ware suchthat the green ware shrinks away from the mold and can be removed easilywithout damage. The cup or liner is porous or only covers part of themold cavity surface to permit the absorbtion of vapors from the dryingceramic mix. The temperatures are limited and the heating time on theorder of a half a minute to drive out the moisture without damage to thepart.

SUMMARY OF THE INVENTION

The invention comprises a method and apparatus to quickly heat aninjection mold surface or portions thereof just prior to the injectionof a foamable plastic resin or molding compound. The apparatus comprisesone or more thin metal surface sheets or bands in the mold cavitycovering those portions of the mold where a smooth surface finish on theproduct is desired. In an alternate form the electrically heated surfacesheet may be plated or sprayed (sputtered) metal of a high resistivityon a layer of thin electrically nonconducting plastic in turn adhered tothe mold cavity surface. A suitable plastic for the non-conducting layermay be selected from the polyimide group.

The thin metal surface sheet is resistance heated about 300° F. in a fewseconds or less by a low voltage high current source of electric powerjust prior to injection of the foamable plastic. The high currentpassing through the metal of high resistivity causes conversion ofelectrical energy into sensible heat very quickly to accomplish quickheating of the mold cavity surface sheet. The low voltage eliminates theneed for substantial electrical insulation between the thin metalsurface sheet and the bulk metal of the mold or die. Where steel moldsand steel surface sheets are employed, the normal oxide coating on thesteel serves as the electrical insulation at the interface of thesurface sheet and the mold.

The thin adhesive layer of plastic in the alternate form acts as anelectrical insulator also without substantially reducing the heattransfer through the layer during the cooling cycle of the moldingprocess. A second advantage to the oxide layer or thin adhesive layer isthe elimination of damage and wear due to the repeated cycling pressuresand temperatures. Crushing of the insulative layer is essentiallyeliminated. The metal surface sheet is typically steel two millimetersor less in thickness backed by the bulk steel mold. The sturdyconstruction thereby results in a more serviceable and longer lived moldcavity construction than the prior art.

To reduce or minimize arcing in the electrical switch gear employed tomomentarily connect the high current low voltage source of electricpower to the surface sheet, a water cooled electric heating pipe bypassis employed as an electrical energy sink during the portion of themolding cycle that does not employ heating of the surface sheet.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross section of a mold modified in accordance with theinvention; and,

FIG. 2 is a partial cross section of the mold of FIG. 1 with a secondmodification according to the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In FIG. 1 the cross section of a mold in closed position is shownincluding male 10 and female 12 mold halves enclosing a mold cavity 14.The female mold half 12 is attached to a fixed plate 16 and the malemold half 10 attached to a traveling plate 18. The fixed plate 16 andtraveling plate 18 are aligned by spaced guide bars 20 and the travelingplate 18 moved by the actuator 22. Also movable on the guide bars 20 isan ejector plate 24 and a plurality of ejector pins 26 for detaching theplastic molded product from the male mold half 10 upon completion of themolding cycle.

Extending into the female mold half 12 are an injection nozzle 28 andshut off pin 30. The nozzle 28 communicates with a sprue 32 in turn incommunication with the mold cavity 14. The holes 34 and 36 indicate aplurality of cooling fluid passages formed in the male and female moldhalves for carrying cooling fluid during the molding process. Theremaining elements of an injection molding machine are well known in theart and the method and apparatus for mechanically actuating the mold andinjecting the foamable plastic resin is not shown or described indetail.

Forming the inside surface of the mold cavity 14 is a thin metallicsheet 38 conforming to the shape of the female mold half 12. The thinmetallic sheet 38 includes an aperture 40 to allow communication betweenthe sprue 32 and the mold cavity 14. The metallic sheet 38 extendsoutside the mold on opposite sides as indicated at 42 and 44 whereelectrical cable attachments 46 and 48 are attached to the sheet 38. Lowvoltage high current electric cables 50 and 52 are fastened to the cableattachments 46 and 48 respectively.

Electric cable 52 is connected through a switch 54 to a low voltage highcurrent source of electric power 56. The cable clamp 58 attaching cable52 to the power source 56 also attaches a third electric cable 60thereto. A metallic heating pipe 62 is electrically connected at 64 tocable 60. Electric cable 50 is attached at 66 to the heating pipe 62 ata location spaced from the attachment at 64. A fourth electric cable 68is attached to the heating pipe 62 at 70 substantially adjacentattachment 66 and extends to a cable clamp 72 providing electricalconnection with the low voltage high current source 56. The attachments66 and 70 may be a common attachment to the heating pipe 62. Duringoperation of the injection molding machine according to the processdescribed below a cooling fluid is passed through the heating pipe 62.

A modification of the apparatus is illustrated in FIG. 2 wherein boththe female 12' and male 10' mold halves are covered with thin metallicsheets 38' and 38" respectively. The ends of the ejector pins 26' extendthrough the sheet 38" as shown. To enable the mold to be opened andclosed, separate cable attachments 48' and 48" and separate cables 52'and 52" are attached to the sheets 38' and 38" respectively. A similararrangement is provided on the opposite side of the mold for cable 50.

The metallic sheet is preferably made of low carbon steel, silicon steelor stainless steel to provide a relatively high resistance to the flowof electric current. The voltage is typically limited to less than 50,however, the current can range up to and if required, exceed 2000amperes. By limiting the voltage, the natural oxide coating on themetallic sheet 38 and the underlying steel mold 12 acts as an adequateelectrical insulator at the interface contact 72 between the mold 12surface and the sheet 38. Alternatively, a thin adhesive layer may beapplied between the mold 12 and the sheet 38 to act as both anelectrical insulator and means to secure the sheet to the mold surface.Such an adhesive layer is formed as thin as possible to minimize theresistance to heat transfer during the cooling portion of the moldingcycle.

In another alternative construction, the metallic sheet may be formed byplating or spraying (sputtering) a metal of high resistivity onto alayer of thin electrically non-conducting plastic in turn adhered to themold 12 surface.

Typically foamed plastic molding compounds require that the surface ofthe mold cavity be in excess of the heat distortion temperature of theplastic to obtain a Class A smooth surface finish. Such a temperature isbetween 350° F. and 550° F. for most common foamable polymers. Byutilizing a metallic sheet 38 of substantially 2 millimeters thickness,the surface temperature of the sheet can be raised 300° F. in a fractionof a second. Thus, the mold surface temperature subsequent to theejection of the previous part, about 140° F., can be raised above theheat distortion temperature in a fraction of a second after the mold isreclosed just prior to injection of the plastic molding compound. Thetime required to reheat the mold surface is therefore a small fractionof the cycle time for the part and the bulk material of the mold halves10 and 12 is not substantially reheated and recooled with each partcycle. The actual heating time for a particular part will depend uponthe size of the part, the required temperature increase and the amperageapplied to the metallic sheet.

Because a very high current load is applied for a short period of timeduring each cycle and then disconnected by switch 54, a bypass load inthe form of heating pipe 62 is provided to minimize arcing in the switchand current surges applied to the current source 56. The electricalresistance of the pipe 62 is high relative to the resistance of thesheet 38, thus closure of the switch 54 effectively creates a shortcircuit path through the sheet 38. The bulk of the current from source56 passes through the sheet 38 with the switch 54 closed. Because of theelectrically insulating layer of oxide between the sheet 38 and the moldhalf 12, the bulk of the electric current passes through the sheet inpreference to the mold half 12 despite use of steel for both the moldhalf and the sheet.

A suitable source of electric power 56 can be a continuous duty arcwelder supply or an alternating or direct current continuous duty motorgenerator set. The capacity of the power source and the amperagecapacity of the switch 54 will be determined by the volume of themetallic sheet 38 and the temperature increase required. The switch 54is closed and opened by the machine control circuitry.

The adhesive plastic layer in the alternative form of the invention maybe a polyimide or other plastic suitable for high temperatures (600° F.)and repeated cyclic heating.

The surface heating method is very quick and therefore imposes anegligible time penalty on the product molding cycle time. The electricenergy utilized to heat the bypass pipe 62 may be recycled as hot waterto maintain the mold temperature or to preheat the foamable plasticresin prior to injection.

Where the particular product shape requires a mold cavity surface withrelatively small radii of curvature or sharp corners, localizedoverheating of the metallic sheet or heater band can occur. To preventsuch localized overheating, the thickness or cross section of the sheetis adjusted by locally thickening the sheet. Thickening can beaccomplished effectively by spraying or plating on additional metal atthe localized areas.

As an example of the process and apparatus, a heater band or metallicsheet is constructed with the following dimensions and property:

    ______________________________________                                        Length =     30    cm                                                         Width =      10    cm                                                         Thickness =  0.2   cm                                                         Resistivity =                                                                              30    micro-ohm-centimeters @ 20° C.                      ______________________________________                                    

For a temperature increase of 200° C. or 392° F., the following timesare typically required with the following amperages:

    ______________________________________                                        1000         amp. requires 0.09 sec.                                          500          amp. requires 0.37 sec.                                          100          amp. requires 0.90 sec.                                          ______________________________________                                    

Thus, the mold cavity surface can be heated very quickly above the heatdistortion temperature of the resin just before or as the injection offoamable resin begins. The bulk of the surface sheet is minimal incomparison with the bulk of the mold halves, thus the newly formedfoamed plastic product can be cooled quickly below the heat distortiontemperature. Very little mass is heated and cooled cyclically in theprocess. The heating time is so short as to not appreciably lengthen thecycle time for the entire molding process cycle. The layer of oxide orthin adhesive plastic between the metallic sheet and the bulk of themold minimizes resistance to heat transfer during the cooling portion ofthe process cycle.

I claim:
 1. A process for quickly manufacturing injection molded foamedresin products with a smooth surface finish comprising the stepsof:closing the injection mold, applying high current low voltageelectric power to a heater band on the surface of the mold cavity tosubstantially increase the mold surface temperature within a matter ofseconds or less in those portions of the mold cavity covered by theheater band, disconnecting the high current low voltage electric powerfrom the heater band, injecting a molten foamable resin into the moldcavity, the foamable resin thereupon expanding against the hot heaterband, reducing the temperature of the resin in the mold cavity below theheat distortion temperature of the resin by cooling the mold and heaterband, and, opening the injection mold and removing the plastic product.2. The process of claim 1 wherein the heater band temperature is rasiedfrom approximately 150° F. to a maximum of 600° F.
 3. The process ofclaim 1 wherein the current applied to the heater band ranges from 100to 3000 amperes.
 4. The process of claim 1 wherein the voltage appliedto the heater band is limited to less than 50 volts and the currentexceeds 100 amperes.
 5. The process of claim 1 wherein at least aportion of the resin is injected prior to disconnecting the electricpower to the heater band.
 6. The process of claim 1 wherein the heaterband is supplied with electric power for less than five seconds permolding cycle.